1. Field of the Invention
The present invention relates to a projection exposure apparatus used when a semiconductor integrated circuit, a liquid crystal display device, or the like is manufactured and, more particularly, to a projection exposure apparatus for performing exposure by a scanning exposure scheme.
2. Related Background Art
When a semiconductor element, a liquid crystal display element, or the like is to be manufactured by a lithographic process, a projection exposure apparatus is used. This apparatus is designed to project a pattern image of a photomask or reticle (to be generically referred to as a reticle hereinafter) on a photosensitive substrate through a projection optical system. As such an apparatus, a projection scanning type exposure apparatus is known, which is designed to simultaneously scan a reticle and a photosensitive substrate through a projection optical system.
As a conventional exposure apparatus of this type, an apparatus having a reflecting projection optical system with X1 magnification is known. In this apparatus, a reticle stage for holding a reticle and a wafer stage for holding a photosensitive substrate (to be referred to as a wafer hereinafter) are coupled to a common movable column, and the reticle and the wafer are scanned/exposed at the same speed. In such a scanning exposure apparatus (mirror projection aligner) with X1 magnification, if a reticle pattern and a reticle pattern image projected on a wafer do not have a mirror-image relationship, an exposure operation is completed by one-dimensionally scanning an integral movable column in the widthwise direction of arcuated slit illumination light while the reticle and the wafer are aligned and held on the movable column. As is apparent, with a projection system with X1 magnification in which a reticle pattern and a reticle pattern image projected on a wafer have a mirror-image relationship, the reticle stage and the wafer stage must be moved in opposite directions at the same speed.
Another conventional scanning exposure apparatus incorporating a refracting element is also known. In this apparatus, while the projecting magnification is increased or decreased with the refracting element, both the reticle stage and the wafer stage are relatively scanned at a speed ratio corresponding to a magnification. In this case, the projection optical system used is constituted by a combination of a reflecting element and a refracting element or by only a refracting element. An example of the reduction projection optical system constituted by a combination of a reflecting element and a refracting element is disclosed in U.S. Pat. No. 4,747,678.
In addition, a method of performing step & scan exposure by using a reduction projection optical system capable of full-field projection is disclosed in U.S. Pat. No. 4,924,257. In this method, the reticle stage for holding a reticle is designed to be movable in both the X direction as a scanning direction and the Y direction perpendicular to the scanning direction. Similarly, the wafer stage for holding a wafer is designed to be movable in both the X and Y directions. As disclosed in U.S. Pat. No. 5,004,348, the wafer stage and the reticle stage of an exposure apparatus based on the widely used conventional step and repeat scheme are also designed to be movable in both the X and Y directions. A conventional scanning exposure apparatus may use the wafer and reticle stages of the above-described exposure apparatus of the step and repeat scheme so as to perform control to synchronously scan the two stages in the X direction as the scanning direction. In this case, while a wafer and a reticle are scanned in the X direction, the wafer stage and the reticle stage are finely moved within the X-Y plane to adjust the positions of the wafer and the reticle in the X and Y directions and the direction of rotation, thereby correcting the position deviation of the wafer relative to the reticle. Both the stages, however, are relatively heavy. For this reason, they are poor in response characteristics and require complicated control. That is, in a conventional scanning exposure apparatus, it is difficult to perform constant speed drive control in the scanning direction and simultaneously perform high-precision control of positioning operations in the X and Y directions and the direction of rotation.
As the above-described scanning exposure apparatus, a projection exposure apparatus based on a scanning exposure scheme designed to perform stitching is known (U.S. Pat. No. 3,538,828). In this scanning exposure scheme designed to perform stitching, exposure light having a predetermined shape is radiated on a reticle, and the reticle and a wafer are synchronously scanned, thereby performing exposure with respect to an area corresponding to the first column on the wafer.
Subsequently, the reticle is replaced or is moved in the second direction perpendicular to the first direction of the illumination area by a predetermined amount. The wafer is laterally shifted (stitching) in a direction conjugate to the second direction of the illumination area. Exposure light is radiated on the reticle again, and the reticle and the wafer are synchronously scanned, thus performing exposure with respect to an area corresponding to the second column on the wafer. With this operation, one shot area, on the wafer, which can be exposed can be further increased. In this case, the moving amount of the wafer in the second direction is set such that the exposure areas of the first and second columns on the wafer overlap each other.
In such an exposure apparatus, high-precision overlapping of patterns and a reduction in illuminance irregularity at the overlapping portion between the areas of the first and second columns are required. However, these requirements are not satisfied by the conventional exposure apparatus.
The following problem is posed even in an exposure apparatus having a regular hexagonal illumination area such as the one disclosed in U.S. Pat. No. 4,924,257.
FIG. 14A shows an illumination area on a reticle in a projection exposure apparatus of a stitching and slit scanning exposure scheme. Referring to FIG. 14A, exposure light from an illumination optical system is radiated on a regular hexagonal illumination area 1 centered on a position A. The illuminance in the illumination area 1 is uniform. By scanning the reticle in the -X direction with respect to the illumination area 1 at the position A at a constant speed V/.beta., the illumination area 1 relatively moves over the reticle along a trace 2A and reaches a position B. The reticle is then moved in the Y direction to relatively move the illumination area 1 over the reticle along a trace 2B, thus causing the illumination area 1 to reach a position C. Thereafter, the reticle is scanned in the X direction at the constant speed V/.beta. to relatively move the illumination area 1 over the reticle along a trace 2C.
FIG. 14B shows an exposure area on a wafer. Referring to FIG. 14B, a regular hexagonal exposure area 3 centered on a position AP is conjugate to the illumination area 1 at the position A on the reticle. The regular hexagonal exposure area 3 has two sides parallel to the Y direction. Letting R be the distance between two opposing vertexes of the regular hexagonal exposure area 3, and W be the distance between two opposing sides thereof, W=3.sup.1/2 R/2. When the wafer is scanned in the X direction with respect to the exposure area 3 at the position AP at a constant speed V, the exposure area 3 relatively moves over the wafer along a trace 2AP and reaches a position BP. In this state, when the wafer is moved in the -Y direction by a distance 3R/4, the exposure area 3 relatively moves over the wafer along a trace 2BP and reaches a position CP. Thereafter, when the wafer is scanned in the -X direction at the constant speed V, the exposure area 3 relatively moved over the wafer along a trace 2CP.
The exposure area 3 which relatively moves along the trace 2AP and the exposure area 3 which relatively moves along the trace 2CP are scanned in the Y direction, i.e., the widthwise direction, such that their isosceles triangle areas are superposed on each other in a connection area 4.
FIG. 15A shows a case where a regular hexagonal exposure area 3 is illuminated with a pulse laser beam from a pulse laser source. Referring to FIG. 15A, the exposure area 3 is an area inscribed in the contour of a circular exposure area 7, of a projection, optical system, located on a wafer. Similar to equation (4) in the second embodiment, if the width of the exposure area 3 in the X direction as a relative scanning direction is represented by W, W=m.multidot..DELTA.L=m.multidot.T.multidot.V where T is the period of pulse emission of a pulse laser source 52 in FIG. 6, .DELTA.L is the distance by which a wafer 14 is scanned in the X direction during one period T in a slit scanning exposure operation, and m is an integer larger than one.
FIG. 15A shows a case where m=8. Assume that an exposure point P0 is located at an edge portion of the exposure area 3 when pulse emission occurs. The exposure point P0 is exposed to a pulse laser beam seven times within the exposure area 3, and is exposed to a pulse laser beam twice at the edge portion. In this case, since the energy exposed at the edge portion is 1/2 that exposed within the exposure area 3, energy corresponding to a total of eight pulses is radiated on the exposure point P0. Energy corresponding to a total of eight pulses is radiated on the exposure point P0 regardless of the X-direction position of the exposure point P0 at the time of pulse emission.
Consider an exposure point through which an area 3a of the right-hand isosceles triangle of the exposure area 3 passes. The distances by which exposure points P1 to P8 shown in FIG. 15A pass through the area 3a of the isosceles are 8.multidot..DELTA.L to 1.DELTA..DELTA.L, respectively. Therefore, when the wafer is scanned in the X direction with respect to the exposure area 3 (the first wafer scanning operation), energy corresponding to eight pulses is radiated on the exposure point P1, and energies corresponding to seven pulses, six pulses, . . . are respectively radiated on the exposure points P2, P3, . . . .
When stitching of the wafer is performed, and the wafer is scanned in the -X direction with respect to the exposure area 3 (the second wafer scanning operation), energies corresponding to 0 to seven pulses are respectively exposed on the exposure points P1 to P8. Therefore, energy corresponding to eight pulses is radiated on the exposure points P1 to P8, similar to the exposure point P0, by performing exposure twice upon stitching, as in the second embodiment.
However, at an exposure point P9 between the exposure points P4 and P5, even if slit scanning exposure is performed twice, radiated energy varies. That is, as shown in FIG. 15B, pulse emission is performed when the exposure point P9 is at a position 8 in the first wafer scanning operation, and pulse emission is performed when the exposure point P9 is at a position 9 in the second wafer scanning operation. Therefore, energy corresponding to nine pulses is radiated on the exposure point P9.
In the case shown in FIG. 15C, in the first wafer scanning operation, pulse emission is performed when the exposure point P9 is at a position 10, and in the second wafer scanning operation, pulse emission is performed when the exposure point P9 is at a position 11. Therefore, energy corresponding to seven pulses is radiated on the exposure point P9. That is, energy corresponding to seven to nine pulses is radiated on the exposure point P9 depending on the timing of pulse emission. Consequently, at the connection portion 4 on the wafer, radiated energy irregularity, i.e., illuminance irregularity, is caused owing to a pulse laser beam.